An endovascular stent graft is designed to exclude the flow of blood to an aneurysm that has been formed within the wall of the aorta. This is achieved by accessing the aneurysm via an artery, usually within the patient's leg, with a system designed to deliver, position and deploy the stent graft so that it bridges and seals off the aneurysm.
A stent graft is a tubular device with walls made from a flexible sheet material, supported by a rigidising frame which is usually formed from super-elastic metal. Some stent graft designs are fixed to the aorta wall by means of barbs or hooks. The rigidising frame maintains the tubular shape of the stent graft, while providing a radial sealing force to create a proximal and distal seal with the aortic wall.
In the thoracic aorta it is becoming increasingly desirable to place the proximal end of the stent graft within the arch of the aorta, sometimes as proximal as between the brachiocephalic artery and the left carotid artery. In some applications, such as the placement of percutaneous heart valves, it is envisaged that the stent graft should be placed at the very origin of the aorta.
Placement of stent grafts in the arch of the aorta is particularly difficult because of the force of blood ejected from the heart. In more distal vessels, the capacity of the aorta between the heart and the landing zone of the device is significant and reduces the maximum velocity of blood experienced by the graft. In the arch of the aorta, the capacity of the vessel between the heart and the landing zone of the stent graft is minimal and hence the stent graft is subject to the full ejection velocity of blood from the heart.
The effect of this force with current devices is usually to turn the proximal end of the device to lift it away from the wall of the aorta, commonly at the inner wall of the arch. Over time, the extent of this turn can increase, reducing the flow of blood in the distal aorta and potential leading to failure of the fixation system of the stent graft. In order to overcome this problem the delivery system employed to place the stent graft must be capable of controlling the angle of the proximal end of the stent graft in the vessel wall.